Here is the question I hear every November: what heater should we buy? By January, the same people are asking a different question: why is the electricity bill so high?

I handle purchasing for a 45-person engineering company. I have managed our facilities budget for six years and I have kept a spreadsheet of every heating-related invoice since 2019. That spreadsheet has cost me enough mistakes. This article is the checklist I now run before buying anything that produces heat.

It is for anyone who buys or approves heating for a workshop, warehouse, office, or retail space. Seven steps. Run them in order. The expensive lessons usually come from skipping Step 1.

Step 1. Define the heating job before you look at heaters

Do not start with the machine. Start with the room and the schedule. If you skip this, you will pick a heater by gut feel, and gut feel is exactly what makes vendors happy.

Write these down before you open a product page:

  • Square footage and ceiling height. A 400 sq ft office with a 9 ft ceiling is not the same as a 400 sq ft workshop with an 18 ft ceiling.
  • Insulation level. Are you heating a plasterboard room with new windows, or a steel building with draughty roller doors?
  • How often doors and loading bays open.
  • Hours you actually need the target temperature, including weekends.
  • Target temperature. Some rooms only need to stay above freezing. Those rooms should not be heated to 21°C.

Then make two decisions: do you need fast warm-up or steady heat, and is the space occupied by people all day or only loaded and unloaded?

Step 2. Understand how the three main options actually work

You cannot compare prices until you can compare what the machine does with energy. Here is the plain English version.

Electric resistance heaters

A fan heater, panel heater, or oil free radiator turns electricity directly into heat. One kilowatt in equals roughly one kilowatt of heat out. No flue, no combustion, no fuel tank, and very little maintenance. The differences are in how they deliver the heat.

Take a Dimplex 3kW flat fan heater. It pulls air over a hot element and blows it into the room. The room warms quickly, so it suits workshops, garages, and anywhere you need heat in a hurry.

A Dimplex oil free radiator has a finned body that warms the surrounding air by natural convection. It takes longer to get going but produces a quieter, steadier heat. That makes it a better fit for offices and rooms where people sit still.

Heat pumps

Since how does a heat pump work is the question that keeps coming up, here is the short answer. A heat pump uses refrigerant and a compressor to pull heat from the outside air and move it inside. One kilowatt of electricity can move around three kilowatts of heat into a building on a mild winter day. That is why heat pumps look so good on paper.

The catch is in the phrase mild day. When the temperature drops to around -10°C, outdoor air contains less heat, so the unit works harder and its efficiency drops. Read the manufacturer’s performance curve at your local winter design temperature, not just the seasonal average. Heat pumps also cost more to install and need regular maintenance.

Diesel heaters

A diesel heater burns diesel in a burner, pushes the combustion heat through a heat exchanger, and blows warm air into the room. The exhaust gas goes outside through a flue. These units are common in workshops and farms because they can produce serious heat without needing a large electrical supply.

But a diesel heater is a system, not an appliance. You have fuel to store, filters to change, a fuel pump, a fan, and a burner that needs service. It also makes noise. When you compare running costs, include every one of those factors and not just the pump price of diesel.

Step 3. Run a total cost of ownership comparison, not a price comparison

Total cost of ownership means every cost from the day the unit arrives to the day you replace it. My checklist forces me to put four numbers in a spreadsheet:

  • Purchase price, delivered.
  • Installation cost, including any electrical work.
  • Expected annual running cost.
  • Expected annual maintenance and cleaning.

For an electric resistance heater, the running cost formula is simple: input kilowatts times operating hours times your price per kilowatt hour.

Here is an example. A 3kW heater running at full output for four hours a day over 60 winter days uses 720 kWh. At $0.16 per kWh, which is roughly the US residential average according to the US Energy Information Administration in early 2025, that is about $115 for the winter. If a thermostat cuts the running time in half, halve that number. Check your own commercial tariff because it will be different.

For a heat pump, replace the input kilowatts with the input at your local design temperature. If the unit moves 3kW of heat for every 1kW it draws, the same heat costs roughly a third of electric resistance. But the upfront cost is higher and you will need a service arrangement.

For diesel, convert fuel to useful heat. Diesel contains about 10 kWh of energy per liter. A good diesel heater might put 8 kWh of that into the room after combustion losses. If diesel costs $0.90 per liter, that works out to roughly 11 cents per useful kWh. At 16 cents per kWh, electric resistance costs more per useful kWh. But that comparison only holds if you ignore maintenance, noise, and the staff hours spent dealing with fuel. I never ignore those.

Use your own local numbers. Your electricity tariff is on your utility bill. Diesel prices are at the pump. Plug them in.

Step 4. Ask what is not included before you ask for a discount

Once you have a quote, ask this exact question: what would make this price go up? Ask it before you ask whether the vendor can do better.

The list of extras is predictable. Delivery to the right door, assembly, mounting brackets, thermostat, wiring connection, dedicated circuit, removal of the old unit, disposal, and warranty labour. If a vendor lists every cost in the first email, that is a green flag. If the vendor shows a low price and says everything else is standard, keep digging.

In 2023, I compared two quotes for the same type of wall-mounted heater. Quote A was $189. Quote B was $222. I almost bought A until I asked for the fully installed price. After shipping, a mounting bracket, a thermostat, and electrical work, Quote A came to $347. Quote B included all of that from the start and came to $314. The more expensive quote was cheaper.

Asking what is not included is the most underrated procurement question in existence.

Step 5. Check installation and safety before the box arrives

This is where radiator covers belong. If a heater sits in a hallway, day-care room, care corridor, or anywhere someone could touch a hot surface, a guard or cover is a legitimate safety feature. But a cover can also stop the heater from doing its job.

Convection heaters rely on air moving past the heating element and fins. Put a solid wooden cover around one and the heat collects inside, the element overheats, and the unit cycles on its thermal cutout. I spent $400 in 2023 learning this after fitting a cover in our reception area. I spent more money in 2024 learning it a second way when we removed the cover and found the heater had been running poorly for months.

If you need a radiator cover for safety, choose one with an open front and plenty of gaps, and check whether the heater manufacturer allows any enclosure at all. Do not design a custom cabinet around a unit that was only approved for open mounting.

Also check clearances and power before the unit is on site. A 3kW heater on a 230V supply draws around 13 amps. That is a serious continuous load. It is not something to hang on a cheap extension lead or a circuit that already supplies lighting and computers. When an electrician says it needs a dedicated circuit, treat that as installation cost, not a vendor upcharge.

One more thing for diesel heaters: the exhaust has to go outside. Buy the flue kit, route it properly, and install a carbon monoxide alarm in the space. A diesel heater with a blocked exhaust is not a budget problem. It is a safety problem.

Step 6. Put controls on everything

A heater without a thermostat is just an expensive way to make a room too warm and then switch off. If the heater has a built-in thermostat and timer, use them. If not, add a controller that can follow your actual schedule.

The heater that runs all night in an empty office is the most expensive heater ever sold. I know because I walked past that same office for two winters before I fixed the schedule.

For a Dimplex fan heater or oil free radiator, the integrated thermostat is not a luxury feature. It is the difference between heating the room to the set point and then maintaining it, versus heating the room until someone remembers to walk over and switch it off. In a commercial space, a wired wall thermostat is usually better than a rotary dial on the unit because people fiddle less.

Controls matter more than most people think. In a moderately insulated room that is occupied on a fixed schedule, a timer alone can cut running time by a third.

Step 7. Buy one, measure, then roll out

Do not buy a fleet of heaters based on a product photo and a spec sheet. Buy one unit, install it in the worst room, run it for a week, and write down what actually happens.

Track these:

  • Actual kilowatt hours used per day, measured with a plug-in energy monitor.
  • Difference between the thermostat set point and the real room temperature.
  • Time it takes to reach a comfortable temperaturein the morning.
  • Complaints about noise, airflow, or the unit switching on and off.
  • Whether the controls stay at the settings you programmed.

If the pilot unit passes, order the rest. If it does not, test a different unit. Pilots also help with the office politics part of the job. When someone says this heater is not strong enough, you can answer with logged data instead of an argument.

Common mistakes to avoid

Here are the mistakes I have personally made or watched other people make, usually more than once.

  1. Buying the biggest heater available and then wondering why the room feels stuffy and the bill is high. Oversized units short-cycle and spread heat unevenly.
  2. Comparing only maximum output numbers. A machine that is advertised as 3kW will use roughly 3kW when running at full, but the control system decides how much of that output you actually need.
  3. Forgetting the electrical supply. Big heaters need clean, dedicated circuits.
  4. Assuming a diesel heater needs no planning. Fuel storage, exhaust routing, ventilation, noise, and carbon monoxide detection are all part of the purchase.
  5. Buying a radiator cover without checking whether it blocks airflow. If you need one for safety, get one with an open design and enough clearance.
  6. Skipping the maintenance routine. Every heater needs a five minute clean at the start of the season. Dust and blocked vents change performance.

Buying a heater is rarely exciting. It is a series of small decisions with a long tail. The checklist does not make you a heating engineer. It makes you a better buyer. If you only copy one habit from this article, copy the question from Step 4: what is not included in that price?